If passage resistance during fluid flow is changed, the pressure of the fluid is changed. In case of a compressible fluid, that is, gas, the variation of the passage resistance just causes pressure variation, but in case of an incompressible fluid, that is, liquid, a water head itself is changed. That is, if the flux of the fluid flowing in a pipe is changed by the drastic opening/closing of a valve or the on/off operations of pumps, pressure variation momentarily occurs, which is called ‘water hammer’. A water hammer wave generated from the water hammer is reciprocated along the pipe to generate noise and vibration, which is transmitted to neighboring structures (pumps, valves, etc.) through the pipe, thereby causing them to be seriously damaged.
As the flux of fluid becomes rapid and the change time of flux becomes short, the water hammer is increased. At this time, the negative pressure is under a saturated vapor pressure of water, thereby forming vapor cavity to cause the water column separation or the collapse of the pipe.
Generally, the flow of fluid in a pipe is largely divided into steady flow that is constantly maintained, irrespective of time, and unsteady flow that is changed in accordance with time, and in this case, the unsteady flow is subdivided into quasi-steady flow wherein the inertia or elasticity of fluid does not give any influence on the movement of the fluid in the pipe and unsteady flow wherein the inertia or elasticity of fluid gives an influence on the movement of the fluid in the pipe.
At this time, the water hammer has a relation with the unsteady flow wherein the inertial effect of the fluid is important and the compressibility effect of the pipe and fluid is very low or does not exist almost, which means rigid-column flow.
The water hammer necessarily occurs during the operation of the system, and so as to reduce the damage caused by the water hammer to a maximum degree, the following five rules should be observed:
Firstly, the momentary change of the flux of fluid through drastic manipulations of a valve and operation/stop manipulations of pumps should be prevented (the prevention of momentary change of flux);
Secondly, the valve and pumps are frequently checked and controlled so as to prevent the increased pressure caused by the water hammer from being raised more than the internal pressure of a system (maintaining the increased pressure below the internal pressure of a system);
Thirdly, if pressure is lowered below the saturated vapor pressure of a fluid, the fluid is changed from the liquid state into a gas state, thereby causing given problems, and accordingly, the pressure variation is noted during air bubbles are generated and disappear to control negative pressure (the prevention of the generation of negative pressure);
Fourthly, in even case where air is introduced into a pipe, if an amount of air introduced is not large and an appropriate flux exist, no problem occurs in the pipe system, but if an amount of air introduced is large and the air exhaust operation is not gentle in the pipe system, the air stays in the pipe, which causes many problems such as the unnecessary consumption of power, the reduction of flow rate, the generation of noise, corrosion and the like (the suppression of the introduction of air into pipe); and
Lastly, if the inside diameter of the pipe in the pipeline is small, the flow rate is severely changed to cause the pressure variation width to be increased upon the generation of water hammer, and therefore, the appropriate pipe size and the longitudinal profile and protection mechanism for the safety upon the installation of the pipeline should be needed (the selection of appropriate pipeline installation and pipe inside diameter).
So as to reduce the water hammer, accordingly, a low speed rotary type screw valve, a valve using air or oil pressure have been developed under the studies continuously made in this field, but they have had other problems, thereby making it hard to conduct stable control. Accordingly, there is a need for the development of a new method for automatically recognizing water hammer and treating the recognized result.
A new water hammer recognizing and controlling method is disclosed in Korean Patent No. 742398 (prior art document 1) issued to the same applicant as this invention, wherein the level and pressure variations of an air chamber are compared with a single set value to recognize water hammer, so that whenever the water hammer is recognized, the on/off operations of an air compressor and an exhaust valve (solenoid valve) of the air chamber should be frequently conducted, which undesirably causes excessive energy consumption in a pipe system where a plurality of pumps are combined and controlled respectively in the on/off operations in accordance with the required capacity.
As mentioned above, the water hammer recognizing and controlling method is disclosed just in Korean Patent No. 742398 (prior art document 1) issued to the same applicant as this invention, and further, there have been disclosed Korean Patent No. 868908 (prior art document 2) wherein a single level of fluid in an air chamber is sensed to control a solenoid valve and an air compressor of a water hammer prevention system. Also, there have been disclosed Korean Patent No. 982683 (prior art document 3) wherein the pressures and levels of a plurality of air chambers are checked to control a water hammer prevention system. However, the prior art documents 2 and 3 do not have any function of recognizing water hammer, and one pressure and level of just one air chamber is sensed to control the water hammer prevention system, so that the control methods in the prior art documents 2 and 3 are excessively sensitive to the water hammer in recent pipe systems controlling the flow through the variable control of a plurality of pumps, thereby undesirably conducting the frequent on/off operations of the air compressor and the exhaust valve, in the same manner as mentioned in the prior art document 1.